This invention relates to a fibroin-coated pigment comprising a carrier pigment in the form of finely divided particles whose surfaces are substantially coated with a film of silk fibroin coagulated and precipitated from a solution thereof (hereinafter referred to as a film of regenerated fibroin) and having great utility in the manufacture of cosmetic preparations, coating compositions and the like, and to processes for producing the same.
Conventionally, pigments for use in cosmetic preparations and coating compositions have several disadvantages. That is, when dispersed in water, such pigments are liable to aggregation and precipitation, and when dispersed in oil, their oil absorption properties are suppressed to such an extent that they are hardly wetted with oil, cannot be dispersed therein satisfactorily, and hence are apt to aggregate. Especially in the case of cosmetic preparations, such pigments remove moisture and grease from the skin and bring about a dehydrated, degreased or dried condition which may roughen the skin. Moreover, they lack adhesion to the skin, spreadability on the skin, smoothness and the like. Thus, the direct use of such pigments can hardly produce satisfactory cosmetic effects. Furthermore, it is said that, when calcium carbonate and the like adsorb moisture thereon, the pH of the skin surface becomes alkaline and tends to roughen the skin.
In order to improve the properties of such pigments, a process for the production of a pigment having a silk-vinyl coating is disclosed in Japanese Patent Publication No. 250/'53. According to this process, a pigment (such as titanium oxide, zinc oxide, kaolin, talc, etc.) is mixed with a methanolic solution of polyvinyl acetate. While the resulting mixture is being stirred, a caustic potash solution containing fibroin is added thereto drop by drop to form and precipitate polyvinyl alcohol by the saponification of the polyvinyl acetate and, at the same time, to coagulate the fibroin by the action of the methanol. The white gelatinous precipitate so formed is heat-treated at 95.degree. C. in the water-methanol-caustic potash system, allowed to cool, separated from the liquid phase, neutralized with hydrochloric acid, washed with alcohol and water, and then dried under reduced pressure.
However, the coated pigment obtained by this process has a coating composed of polyvinyl alcohol and fibroin. Though heat-treated (or crystalized), the polyvinyl alcohol tends to swell and dissolve in water. Accordingly, when water-base paints and cosmetics (such as face-powder fluid, cream and lotion) containing this coated pigment are stored for a long period of time, the polyvinyl alcohol may swell or dissolve. Moreover, when this coated pigment is dyed, the polyvinyl alcohol may dissolve in the dyeing solution and cause the surfaces of the pigment particles to be exposed partially. In consequence, the resulting coated pigment lacks in dispersibility in water and evenness of dyeability, and the final products (e.g., cosmetic preparations) containing it are poor in such properties as adhesion, spreadability, storage stability, etc. Furthermore, the fibroin present in the coating is denatured and hardened to a considerable degree because of the use of alkali in the production process and the heat treatment in the presence thereof, and it is not intimately mixed with the hard polyvinyl alcohol. Accordingly, the coating is apt to break as a result of intensive blending during the manufacture of coating compositions and cosmetic preparations (such as face powder and cheek rouge) in powder form. Moreover, this coated pigment is poor in such properties as feeling, adhesion to the skin, spreadability on the skin, oil absorption, dispersibility in oil, color fastness, etc., and is liable to undergo peeling-off of the coating and bleeding of the dye under the influence of water or sweat. In addition, the above-described process involves the use of large amounts of strong alkali (which is apt to denature fibroin) and methanol (which is toxic and dangerous) and requires a considerable number of troublesome operations, which makes it difficult to put this process into practice on an industrial scale.
In Japanese Patent Publication No. 299/'52, a formulation for the production of a face powder is described as Example 4. According to this formulation, a portion of 1.385 g of a pigment is intimately mixed with 5.0 g of a colloidal solution of fibroin (as a binder). Then, the remainder of the pigment and an appropriate amount of coloring matter are added to and blended with the resulting mixture. Finally, 35 g of perfume is incorporated therein to obtain a face powder.
However, it is well known that, when such a small amount of binder (i.e., a colloidal solution of fibroin) is mixed with a large amount of pigment, the binder is only partially deposited on the surface of the pigment particles, and not over the entire surfaces of the pigment particles. Even if the pigment having the binder deposited thereon is dried (in air or by heating), the structure in which the surfaces of the pigment particles are uniformly coated with a film of fibroin is not created and, moreover, the fibroin is scarcely converted into that type of fibroin having the .beta.-configuration. Accordingly, this fibroin-loaded pigment and the face powder containing it are insufficient in such properties as adhesion to the skin, spreadability on the skin, and oil absorption, and the fibroin present on the surfaces of the pigment particles tends to cohere or peel off under the influence of water or sweat. Thus, they can hardly produce good cosmetic effects.
In addition, silk powder (consisting of fibroin alone) has been used in cosmetic preparations in powder form, because of its silk-like feeling, good gloss, high ultraviolet-absorbing powder, moderate hydrophilic-lipophilic balance, good adhesion to the skin, and the like.
However, silk powders produced by conventional grinding techniques, silk powders in granular form, and the like do not permit microscopically intimate mixing. Moreover, in order that the covering power and gloss of the pigment (e.g., talc, titanium oxide, mica, etc.) may be retained to a full extent, they must be used in an amount of no more than several percent. Accordingly, it is impossible to make good use of the excellent properties possessed inherently by fibroin. In order to utilize the properties of fibroin, the use of a microscopically intimate mixture in which the surfaces of finely divided particles of a pigment are masked (or coated) with fibroin particles is ideal. However, this is utterly infeasible in view of the particle shape and particle size of conventional silk powders.